The use of pressure-resistant reinforcing spiral hollow ribs on underground synthetic resin pipes is known in the prior art. However, these ribs have always appeared on the external circumferential surface of the pipe, partly because these underground pipes were used mostly for water supply and drainage, but also because of circumstances in the manufacturing techniques for general spiral-ribbed pipes.
The use of underground resin pipe in other applications has been rapidly expanding. For example, these pipes are being used as protecting tubes for electric lines. In addition, these pipes have continued to be used as well pipes and manhole pipes and also to join conventional underground pipes.
Known resin pipes may be used for the purposes above state. However, there exists a number of problems in such use. The problems arise in part due to the fact that the reinforcing ribs are located on the external surface of the pipe. Certain conditions in the soil may lead to a lack of evenly distributed soil pressure along the outer surface of the pipe. For example, in situations where the outer ribs come in contact with foreign objects which are greater in diameter or width than the distance between adjacent ribs, the soil pressure is not uniform on the outer surface of the pipe. It becomes difficult to distribute the soil pressure evenly on the ribs and the portions between the ribs. The result may be intense pressure at the crest of the ribs while the area between each rib is free from contact with the soil or a foreign object. This has lead to damage to the rib portions or depressions in the wall of the pipe which would not have occurred had there been uniform soil pressure exerted on the exterior of the pipe.
Where the known resin pipes have been used as protecting tube for electric lines, problems have arisen upon inserting the electric lines into the tube. The friction coefficient is very large since the inner wall of the tube is substantially straight and has an even surface. This also makes insertions of electric lines through a long tube difficult. Such are exemplary disadvantages which accompany the conventional pipes.